1. Field of the Invention
The field of the invention is that of satellite telecommunication systems.
2. Description of the Prior Art
A conventional satellite telecommunication system includes a constellation of non-geostationary satellites (for example 64 satellites in low Earth orbit), a plurality of gateway stations and a plurality of fixed terminals. Each gateway station is associated with a separate geographical cell ("spot") and is connected to a terrestrial telecommunication network. Each terminal communicates via the satellites with the gateway station of the cell in which it is located (and consequently with the terrestrial telecommunication network). When it is active in relation to a gateway station, each satellite receives information from the gateway station that it transmits to the fixed terminal or terminals in its coverage area. When it is active in relation to a gateway station, each satellite transmits to the gateway station information that it receives from each fixed terminal in its coverage area. A satellite can be active in relation to a gateway station only if it is visible from the gateway station.
To be more precise, the invention concerns a method of locating a fixed terminal of a satellite telecommunication system of the above kind.
It is necessary to know the position of each fixed terminal of the system for various reasons. For example, this enables synchronization of the fixed terminals to prevent or to reduce mutual interference between signals transmitted by different fixed terminals when received by the satellites. It also enables the charging area to which each fixed terminal belongs to be determined.
Note that the terminal referred to here is a fixed terminal. It is therefore located when initially installed or each time it is installed again if the terminal is moved (for example from one house to another).
A first prior art solution to the problem of locating a fixed terminal consists in providing in the fixed terminal a receiver of one of the existing location systems such as the Global Positioning System (GPS) or the Global Navigation Satellite System (GLONASS).
This first prior art solution has the major disadvantage of being very costly. A GPS or GLONASS receiver is too costly compared to a "consumer" fixed terminal. Also, because the fixed terminal is located only when it is installed, adding a dedicated location receiver to the fixed terminal represents an investment out of all proportion to the use that is made of it.
A second prior art solution is to employ the existing infrastructure (in particular the fixed terminal to be located and certain satellites of the constellation) and an Angle Of Arrival (AOA) or Time Difference Of Arrival (TDOA) location technique known in itself.
Using the AOA technique the position of the terminal is determined by combining angles of arrival of signals from different satellites. Using the TDOA technique the position of the terminal is determined by combining time differences of arrival of signals from different satellites.
Unlike the first prior art solution discussed above, this second prior art solution does not necessitate any dedicated location receiver (of the GPS or GLONASS type).
Nevertheless, whichever location technique is used (AOA or TDOA), this second prior art solution is difficult to put into practice. The AOA technique necessitates precise referencing of antennas in azimuth and in elevation and the TDOA technique necessitates synchronized satellite transmitters. These various conditions are difficult, if not impossible, to comply with without significantly increasing the cost of the system. Once again it should be noted that because it is effected only on installation, the location of the fixed terminal must not represent an excessive investment.
An objective of the invention is to overcome the drawbacks of the prior art.
To be more precise, one objective of the present invention is to provide a method of locating a fixed terminal of a satellite telecommunication system that is simple to put into practice and low in cost.
Another objective of the invention is to provide a method of the above kind that does not require a dedicated GPS or GLONASS location receiver in the fixed terminal.
Another objective of the invention is to provide a method of the above kind employing the existing infrastructure of the system without imposing draconian conditions on the fixed terminal (no precise azimuth reference) or on the satellites (which do not need to be synchronized).